Creation of environment-conscious biodegradable plastics has attracted attention as part of constructing sustainable society. Poly(3-hydroxyalkanoates) (PHAs) produced by microorganisms such as Ralstonia eutropha, which have both thermoplasticity and biodegradability, have been studied for the application as biodegradable plastics and have already been put to practical use in part.
It is important to construct a less expensive production system and breed microorganisms which can freely produce biodegradable plastics having desired physical properties in order to put biodegradable plastics to practical use more widely. In order to achieve the above, various attempts have been carried out such as a search for a new type of biodegradable polyester synthase, an increase in the production volume of the enzyme by genetic-engineering techniques and metabolism-engineering alteration of the biosynthesis path way in cells.
For example, when a biodegradable plastic is a copolymer, various physical properties of the biodegradable plastics can be controlled by the compositional ratio of monomers. However, a technique to effectively alter the compositional ratio of monomers in a copolymer has not yet been established and so biodegradable plastics having desired physical properties have not yet been obtained.
Recently, an evolutionary-engineering technique has been used to alter the properties of enzymes. The evolutionary-engineering technique means the engineering use of Darwin's principle of evolution. Specifically, it is a method for acquiring enzymes with desired properties by rapidly carrying out a step, in vitro, comprising artificially inducing mutations in a gene encoding a target enzyme, selecting a gene encoding the enzyme which is altered to a desired activity among numbers of mutant genes and amplifying the selected gene. This method has been applied to the alteration of enzymes for detergents and the like, and several successful examples have already been reported. However, applications to alteration of enzymes in the production of biodegradable plastics are not known.
Thus, it is an object of the present invention to provide a method for producing a biodegradable polyester capable of controlling various physical properties thereof, a method for producing a biodegradable polyester having desired physical properties, a biodegradable polyester obtained by the method and a poly(3-hydroxyalkanoate) biosynthetic mutant enzyme capable of producing the biodegradable polyester having desired physical properties.